Demo - widberg/fmtk GitHub Wiki

Despite being unlisted, the FUEL demo on Steam can still be installed for free even if you never owned it. The "Install" button on the SteamDB page for FUEL - Demo will direct you to steam://install/12850, which your browser will open with Steam to install the demo.

Alternatively, you can use SteamCMD. After setting up SteamCMD and logging in, you can install the demo using the app_update 12850 validate command.